The invention concerns a method for determining the spatial distribution of magnetic resonance signals from a predetermined imaging area that consists of at least one extended and interrelated region within a volume under investigation of a nuclear magnetic resonance apparatus, wherein nuclear spins are excited by multi-dimensional RF-pulses using magnetic field gradients and an RF transmitting antenna means with at least one transmitting element, wherein    in a definition step, a k-space trajectory, which is to be generated through magnetic field gradient switching and passed through during excitation, and a phase encoding scheme with phase encoding steps are determined for spatial encoding,    in a preparatory step, the amplitude and phase behavior with time of the RF pulses to be irradiated for exciting the nuclear spins is calculated for each transmitting element of the transmitting antenna means for the selected k-space trajectory,    in an execution step, nuclear spins are excited within the volume under investigation during each phase encoding step through a multi-dimensional RF pulse, phase encoding takes place according to the phase encoding scheme, and magnetic resonance signals are acquired using a receiver antenna means,    in a reconstruction step, two- or three-dimensional spatial distribution and/or spatial allocation of the magnetic resonance signals and/or values derived therefrom are calculated from the magnetic resonance signals acquired in all phase encoding steps, and    in a display step, the results of reconstruction and/or one or more values derived therefrom are stored and/or displayed.
A method of this type is disclosed in [4]. In this conventional method, spatial encoding is performed in a period subsequent to excitation.
Magnetic resonance imaging (MRI), which is also called magnetic resonance tomography (MRT), and spatially resolved magnetic resonance spectroscopy (MRS), which is also called spectroscopic imaging (SI), chemical shift imaging (CSI) or multi-voxel localization MRS, are widely used techniques for non-destructive imaging of the inside of an object under investigation, and are based on the spatially resolved measurement of magnetic resonance signals from the object under investigation. The object under investigation is exposed to a substantially static and homogeneous magnetic basic field within a basic field magnet, and the nuclear spins contained therein are consequently oriented with respect to the direction of the basic field, generally selected as the z-direction of a magnetic coordinate system. In an MR investigation, the nuclear spins of the object under investigation, which are oriented in this fashion, are excited through irradiation of electromagnetic radio frequency (RF) pulses using one or more RF transmitting antennas, to perform precession motions, the frequencies of which are proportional to the local magnetic field strengths. In the MRI and SI methods, which are generally used today, the precession motions of the nuclear spins are superposed with a spatial encoding for all spatial directions to be spatially resolved, through time-variant superposition of magnetic gradient fields Gx, Gy, Gz, which are generated by a gradient system. This spatial encoding is usually described by a scheme in a spatial space through Fourier transformation and spatial conjugation: the so-called k-space. The transverse component of magnetization associated with the precessing nuclear spins induces voltage signals in one or more RF receiver antennas that generally surround the object under investigation. Time-variant magnetic resonance signals are generated by pulse sequences that contain specially selected sequences of RF pulses and gradient pulses, in such a fashion that they can be converted into corresponding spatial images. This is realized in accordance with one out of a large number of well-known reconstruction techniques after acquisition, amplification, and digitization of the RF signals using an electronic receiver system, processing thereof using an electronic computer system, and storage in two- or multi-dimensional data sets. The pulse sequence that is used typically contains a sequence of measuring processes, which are also called phase encoding steps, in which the gradient pulses are varied in accordance with the selected localization method in correspondence with the phase encoding scheme that is used.
One substantial precondition for spatially accurate imaging of the magnetic resonance signals of the object under investigation is that the technical imperfections of the MR measuring system can be neglected or the deviations from the ideal behavior are known and can be correspondingly corrected.
In magnetic resonance imaging and spatially resolved magnetic resonance spectroscopy, spatial localization is usually obtained either by Fourier encoding or spatially selective excitation [1, 2].
In Fourier encoding, the nuclear spins to be investigated are simultaneously excited in the entire volume under investigation and spatially localized through imposition of a spatially dependent phase or frequency encoding of their precession motion. This imposition of spatial encoding is realized by gradient pulses in a phase encoding period subsequent to excitation, in which the phase of the precession motion is changed in dependence on the location, and also during signal read-out through application of a read gradient, thereby obtaining spatially dependent modulation of the precession frequency. Both encodings are usually performed according to an encoding scheme which permits determination of the spatial distribution of the magnetic resonance signals using Fourier transformation.
Spatially selective excitation is a technique which is widely used in magnetic resonance imaging, and is utilized to spatially limit transverse magnetization generated during excitation, and/or to spatially vary its amplitude and phase in the excitation volume. For slice selection, which is the most frequent case of selective excitation, the excitation volume is reduced to a predetermined slice. In volume-selective MR spectroscopy (volume-selective spectroscopy VSS), the selection of an area under investigation, which is generally small compared to the object under investigation, is usually also based on slice-selective excitation and refocusing pulses, wherein spatial selection is successively carried out only in one spatial direction, in each case, using a corresponding gradient pulse.
MRI and MRS methods were also developed for accelerating multi-slice acquisitions, in which several substantially parallel slices are simultaneously excited with different phase encoding in several phase encoding steps, their magnetic resonance signals are acquired, and the signals are allocated to the respective excitation slice through suitable data reconstruction, e.g. Hadamard transformation [3].
Multi-dimensional selective excitation using multi-dimensional RF pulses [4, 5], in which the excitation volume is limited in more than one direction or the excitation is modulated in more than one direction, also produced numerous applications, e.g. excitation of a small, three-dimensional volume or also of several volumes simultaneously within one substantially larger object under investigation for spatially resolved spectroscopy, imaging of a selectively excited “region of interest” (ROI) with reduced field of view (FOV) in order to reduce the measuring time, excitation of special volumes that are adjusted to the structures of the object under investigation, or also echo-planar imaging with reduced echo train lengths. The amplitude and phase modulation during excitation may also be used to compensate for disadvantageous effects of an inhomogeneous B1 field of the RF antennas that are used for transmission. This is an application that has become immensely important due to the strong increase in high-field MRI systems [6].
In other conventional MRI and MRS methods, a few spatially separated areas under investigation are simultaneously selectively excited using multi-dimensional excitation. With this excitation, the magnetic resonance signals are superposed with phase encoding using a suitable encoding scheme to permit separation of the signals with respect to their area of origin, thereby simultaneously acquiring the magnetic resonance signals of all areas under investigation [7].
One decisive disadvantage of the conventional MRI and MRS methods for determining the spatial distribution of magnetic resonance signals within an extended and interrelated region on the basis of spatial encoding is the fact that at least part of the spatial encoding is performed in a phase encoding period subsequent to excitation. The time required therefor can be reduced only to a limited degree due to technical and/or physiological limits of the gradient strengths that can be used and/or the switching times, which delays signal acquisition. For spatially resolved measurement of magnetic resonance signals with very short relaxation time, it is i.a. advantageous and in some practical applications absolutely necessary to largely eliminate this delay. Inclusion of spatial encoding of an interrelated region into the excitation period has been realized up to now for only one spatial dimension. This offers no fundamental solution, in particular, for spectroscopic investigations, in which the signals are acquired without application of a spatial encoding gradient.
One further disadvantage of the above-mentioned MRI and MRS methods is the fact that each phase encoding step is performed with different magnetic gradient pulses that generally differ at least with respect to amplitude and/or duration. Due to technical-physical imperfections in the generation of the gradient pulses, the phase changes of the precession motion of the nuclear spins caused with each phase encoding step will generally contain different errors, which results in spatial inaccuracies in Fourier decoding.
Multi-dimensional excitation using multi-dimensional RF-pulses has hitherto only been used for spatial encoding of separate regions, wherein, if desired, spatial resolution within the individual areas is only obtained through classical spatial encoding subsequent to excitation, which means that the aim to minimize the delay of signal acquisition is not met.
A further aspect of the technical progress of recent years has proven to be advantageous for the practical use of multi-dimensional RF pulses. In the past, spatially selective excitation was generally carried out using one individual RF transmitting antenna with a substantially homogeneous transmitting field (B1 field) in combination with the gradient system. Inspired by the success of parallel imaging, in which the signals are acquired using an arrangement of several RF receiver antennas, which is also called an antenna array in technical literature, in the meantime one has also started to use such arrays for transmission in selective excitation. This allows partial replacement of spatial encoding of the excitation locations, which is realized in selective excitation analogously to acquisition through variation of gradient fields by so-called sensitivity encoding, thereby reducing the length of the excitation pulses. This means that the information is utilized that is contained in the various spatial variations of the transmitting fields of the individual array elements, which are also called transmission profiles below [8, 9].
One of the basic questions concerning the use of spatially selective excitation is the determination of the RF pulses which must be irradiated by the transmitting antenna means in order to generate the desired excitation pattern in combination with the gradient-generating k-space trajectory. In the article “A k-space analysis of small tip-angle excitation” [4], Pauly et al. describe a method for single-channel, spatially selective excitation, with which the sought pulse shape B1(t) can be calculated based on a mathematical analogy of selective excitation with Fourier imaging substantially through Fourier transformation of the desired excitation pattern and scanning of the Fourier transform along the predetermined k-space trajectory. Katscher et al. extended this calculation method for the case of an antenna array with several independent transmitting channels [2].
It is the object of the present invention to provide a method for determining the spatial distribution of magnetic resonance signals from at least one predetermined extended and interrelated region within a volume under investigation, which considerably reduces the delay time between excitation of the nuclear spins and acquisition of the magnetic resonance signals, and ensures that nuclear spins with very short transverse relaxation times can be used for spatially resolved measurement.